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Pyronian

Hot giant planets

Pyronian
Image from AstroChara
Pyronian worlds are brilliantly illuminated on the starward hemisphere, and the nightside is pinkish from the combination of blackbody radiation and absorption from Na/K/H2O, similar to brown dwarfs.
In NoLWoCS, Pyronians (prefix: Pyro-; eg. PyroNeptunian, PyroJovian) are hot giant planets, with tropopause temperatures between 800 K, below which potassium chloride clouds nucleate and the planet becomes a Zestonian, and 2100 K, above which the high temperature condensate clouds dissipate and the planet becomes a Calidian. Pyronians are characterized by the presence of clouds of silicate, as well as aluminium oxide (Al2O3) and titanium dioxide (TiO2) to a lesser extent, together producing bright clouds, alongside alkali metal and water absorptions producing dark blue colour. On Pyronians receiving sufficient high energy radiation, high temperature photochemical haze may form, especially at lower temperatures below 1300 K. Additionally, the nightsides of Pyronians are hot enough to perceptibly glow magenta due to a combination of blackbody radiation and absorption.

Many Pyronians orbit very close to their star, and hence are tidally-locked to their parent star, completing an orbit in a few days. This produces strong temperature contrasts between the dayside and the nightside, as well as powerful wind current blowing across the planet. Silicate clouds may form on the nightside and dissipate when blown to the dayside, for example. The combination of tidal-locking and fast rotation also favours production of giant cyclones in both hemispheres on the dayside.

Pyronians are rare among mature Jovians, and even rarer among mature neptunians, as the temperature is sufficient to evaporate their hydrogen-helium envelope, turning them into terrestrial planets. These planets generally form further out, and either migrate in through interactions with the protoplanetary disc, or through high eccentricity migration: either scattered in by gravitational interactions with other bodies or by von Zeipel-Lidov-Kozai mechanism, followed by tidal circularization. On the other hand, young giant planets usually form hot, and some can become hot enough to belong to this type regardless of their distance to the star as a result.

Lower mass Pyronians generally have larger radii compared to their cooler counterparts, as the high temperature inflates their atmospheres. The atmospheric loss rates are also enhanced; a Pyronian's atmosphere may become so inflated that it fills the entirety of the planet's Roche lobe, and a faintly-visible tail of escaping atmospheric gas can form, pointing away from the star.

HD 189733 b - Blueglass
Image from Steve Bowers
Blueglass - a tidally-locked epistellar jovian planet, is hot enough at the epistellar point to produce blue-gray silicate clouds
Millennium2
Image from Steve Bowers
Licchavi, a hot epistellar Jovian world with unusual spiral rings caused by a recent encounter with a large comet. The dark clouds on this world are composed of alkali metals
 
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Development Notes
Text by AstroChara
John M. Dollan in his Planet Classification List
Initially published on 24 October 2001.

Overhauled 2025-10-14 by AstroChara
 
 
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